It has been conventionally known that a semiconductor element such as a light emitting diode is encapsulated by a resin.
A method for producing an optical semiconductor device has been proposed (ref: for example, the following Patent Document 1). In the method, for example, an encapsulating sheet having a substrate sheet and a silicone resin layer laminated below the substrate sheet is disposed on a board mounted with a light emitting diode and next, the light emitting diode is embedded and encapsulated by the silicone resin layer. Thereafter, the encapsulating sheet is heated at 120 to 250° C., the silicone resin layer (an encapsulating layer) is cured, and subsequently, the substrate sheet is peeled from the encapsulating layer to produce the optical semiconductor device.